Lg TVs do occasionally nag you about connecting to the internet but are otherwise fine completely disconnected.

At least with a TV you can actually turn off the WiFi and just not connect a network cable to it. In my case the TV periodically tries to connect and prompts me to check the network settings, so I’m fairly sure its not trying to randomly connect to an open WiFi network. YMMV I suppose. A decent workaround would be to set up a diff router or ssid and just blackhole that network from getting online at all. But that can be more technical than some people are comfortable with.

Don’t connect the TV to the internet. Buy a settop box like a shield TV and stream through that.

Container tabs do sync in recent versions of firefox

https://blog.mozilla.org/security/2020/02/06/multi-account-containers-sync/

Try Firefox relay. It can set you up with a randomly generated email address that will forward to your regular email.